Vacuum waste systems are used aboard ships, aircraft, trains, in prisons and in high use public facilities not adequately served by traditional waste systems. Vacuum systems reduce the physical amount of waste generated and conserve the limited storage and on-board treatment capacity in these environments.
Vacuum systems have several advantages over traditional sewer systems. One advantage is that typical vacuum systems use less water per flush (approximately one pint versus one-and-one-half to three gallons in traditional systems). Another advantage is that the vacuum system pipes are not restricted to a linear, horizontal configuration since it is vacuum (typically 12-16 inches Hg at 25 feet/second) rather than gravity that is used to convey waste to a holding tank. This feature permits vacuum pipes to include bends, turns, and vertical sections. In turn, this allows the network of pipes to conform to the layout of the vehicle, rather than the vehicle having to conform to the layout of the waste system.
A major disadvantage of typical vacuum systems, however, is that the bends, turns, and vertical sections of the pipes accumulate scale faster than linear or horizontal pipe sections. This presents a confounding maintenance problem after even moderate periods of use. As waste is pulled through the pipe, it causes a sheeting action of water/suspended solids to form along the pipe walls due to surface tension. Even though the solenoid valves at each toilet/urinal are only open for one to two seconds, the resulting air flow is significant enough to evaporate most of the liquid clinging to the pipe walls or bends. The result is formation of an evaporative scale composed of mineral content, primarily in the form of calcium and magnesium carbonate, from the source water and any dissolved mineral content of the waste. Additionally, a portion of the macerated biological solids that were suspended in the wastewater also accumulates on the evaporative scale, thus binding the scale together and on the pipe walls. Vacuum leaks at joints, plugs, or diaphragm valves result in accelerated scale accumulation due to increased and often continuous air flow through the pipe.
Traditional means of cleaning vacuum waste pipe systems, such as hydroblasting with high pressure water or mechanical cleaning with "snakes" or augers, are difficult and are not completely satisfactory.
While hydroblasting can remove loose debris, its disadvantages are that it only works with straight pipe runs, the waste water has to be removed at an outlet other than the access point, it is a lengthy and messy process, and substantial amounts of hard scale still remain on the pipe walls. Mechanical cleaning removes only the very loose debris in the center of the pipe without removing any of the very hard scale that significantly reduces the carrying capacity of the pipes. An additional disadvantage of both hydroblasting and mechanical cleaning means is that both require almost complete dismantling of the system to create access for cleaning and extended periods of down time for the system.